Correlation between ferroelectricity and torsional motion of acetyl groups in tris(4-acetylphenyl)amine observed by muon spin relaxation

TL;DR

This study reveals that the ferroelectric switching in TAPA is driven by local torsional motions of acetyl groups, demonstrated through muon spin relaxation experiments and supported by density functional theory calculations.

Contribution

It is the first to connect molecular torsional dynamics of acetyl groups with ferroelectric behavior in an organic compound using muon spin techniques.

Findings

Torsional motion of acetyl groups increases with temperature.

Spin relaxation rates correlate with atomic displacement parameters.

Structural transition is driven by local molecular motion of acetyl groups.

Abstract

It is demonstrated by muon spin relaxation and resonance experiments that the switchable spontaneous polarization of the organic ferroelectric compound tris(4-acetylphenyl)amine (TAPA) is governed by the local molecular dynamics of the acetyl group. The implanted muon forms paramagnetic states which exhibit longitudinal spin relaxation due to the fluctuation of hyperfine fields exerted from unpaired electrons. The first-principle density functional theory calculations indicate that these states are muonated radicals localized at the phenyl group and on the carbon/oxygen of the acetyl group, thereby suggesting that the spin relaxation is dominated by the random torsional motion of acetyl group around the CC bond to the phenyl group. The stepwise change in the relative yield of radicals at $T_0\approx 350$ K and the gradual increase in the spin relaxation rate with temperature ($T$) indicate that the torsional motion is significantly enhanced by thermal excitation above $T_0$. This occurs concomitantly with the strong enhancement in the atomic displacement parameter of oxygen in the acetyl group (which is non-linear in $T$), indicating that it is the local molecular motion of the acetyl groups that drives the structural transition.